The present invention relates generally to polyolefin foams and, more particularly, to extruded polyethylene or polypropylene foam sheets bonded to a foam sheet comprising a blend of polypropylene and homogeneous ethylene/alpha-olefin copolymer.
Polyolefin foams, particularly polyethylene foams, and methods for manufacturing such foams are well known in the art. See, e.g., U.S. Pat. No. 5,348,984 (Lee), U.S. Pat. No. 5,462,974 (Lee), and U.S. Pat. No. 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE).
While polyethylene (PE) possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of PE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications. For example, PE foams are commonly used to manufacture a type of flotation article known as a xe2x80x9cbodyboard,xe2x80x9d which is a generally planar foamed structure of various shapes and sizes that allows individuals to be carried forward by oceanic waves as the waves break near the shore, i.e., to ride or xe2x80x9csurfxe2x80x9d the waves. This is commonly accomplished by grasping the edges of the bodyboard with a portion of the upper body positioned above, and often in contact with, the upper surface of the board. Individuals engaging in such activity generally prefer that the bodyboard be as stiff as possible as this allows the individual to maneuver the board to control somewhat the individual""s direction of travel relative to the wave surface upon which the individual is xe2x80x9csurfing.xe2x80x9d Too much flexure in the bodyboard detracts from this ability to maneuver the board along the wave surface. Accordingly, efforts have been made to increase the stiffness of the foam used to make bodyboards.
One possible solution would be to construct the bodyboard from polypropylene (PP) foam, which is typically stiffer than PE foam. However, PP foam has generally been found to be overly stiff and brittle, resulting in a bodyboard that is too easily damaged (e.g., gouged) and difficult to firmly grasp and handle, particularly when wet. In addition, PP foam has a surface texture that is uncomfortable against the skin, especially when rubbed against the skin as generally occurs during bodyboarding.
Another possibility is to form a composite structure that includes a PE foam sheet bonded to a PP foam sheet. The PE foam sheet would provide comfort, grippability, and durability, while the PP foam sheet would provide increased stiffness and performance. Unfortunately, PP and PE foams are not chemically compatible and will not readily adhere to one another. Delamination is therefore a significant problem for PE foam/PP foam laminates.
It may be possible, in certain cases, to use special chemical adhesives (i.e., glues) to bond PP and PE foams. However, such adhesives are expensive and the process of coating and bonding is both tedious and expensive. The process involves slow production steps and the adhesives contain volatile organic solvents that are undesirable for release into the air. Extra equipment, therefore, is generally needed to recover the solvents, thus further adding to the expense and complexity of the adhesive coating process.
Accordingly, a need exists in the art for an economical and practical means for improving the stiffness of PE foam, e.g., by bonding PP foam or PP-containing foam to PE foam with sufficient strength that the resultant composite structure is suitable for commercial use, such as for bodyboards or other watersport applications. Such composite structures would also be beneficial in other end-use applications, such as packaging, by providing enhanced cushioning and shock-absorption to packaged articles, e.g., computers, during shipment. A need also exists in the art for a means for improving the brittleness of PP foam.
That need is met by the present invention, which, in one aspect, provides a foam sheet comprising a blend of polypropylene and homogeneous ethylene/alpha-olefin copolymer, wherein the polypropylene is present in the blend at a weight percentage ranging from 70 to 95 and the homogeneous ethylene/alpha-olefin copolymer is present in the blend at a weight percentage ranging from 5 to 30. The inventor has discovered that homogeneous ethylene/alpha-olefin copolymer blends well with polypropylene such that a high quality foam can be made from such blend. As compared to a foam made from polypropylene alone, a foam made from a blend of PP and homogeneous ethylene/alpha-olefin copolymer is less brittle while still retaining a high degree of stiffness. In addition, PP-homogeneous ethylene/alpha-olefin copolymer blend foam in accordance with the present invention has surprisingly been found to provide excellent adhesion to both PE foam and to PP foam without the need for adhesives.
Accordingly, another aspect of the present invention is a foam composite structure, comprising:
a. a first foam sheet comprising polyethylene; and
b. a second foam sheet comprising a blend of polypropylene and homogeneous ethylene/alpha-olefin copolymer, wherein the first and second foam sheets are bonded together at a bond strength of at least about 2 lbf/inch.
A further aspect of the invention pertains to a foam composite structure, comprising:
a. a first foam sheet comprising polypropylene; and
b. a second foam sheet comprising a blend of polypropylene and homogeneous ethylene/alpha-olefin copolymer, wherein the first and second foam sheets are bonded together at a bond strength of at least about 2 lbf/inch.